Transmission shifting with speed dither and torque dither

ABSTRACT

A system and method of dithering speed and/or torque for shifting a transmission of a vehicle having an engine, a reversible, variable displacement hydraulic motor/pump which can be driven by the engine, a hydraulic accumulator supplied by said motor/pump, and at least one reversible hydraulic driving motor for propelling the vehicle supplied with fluid by the hydraulic accumulator and/or by said motor/pump operating as a pump. A transmission unit connects the engine with the variable displacement hydraulic motor/pump during a first mode of operation (city mode) and connects the engine to a vehicle drive wheel during a second mode of operation. The system utilizes stored hydraulic energy to dither the output of the driving motor in order to achieve quick and smooth shifts between city and highway mode, or between various ranges within the city mode.

RELATED APPLICATIONS

This application is a continuation of U.S. patent application Ser. No.12/268,100, which claims the benefit of U.S. Provisional Application No.60/986,303 filed Nov. 8, 2007, and U.S. Provisional Application No.60/986,306, filed Nov. 8, 2007, all of which are hereby incorporatedherein by reference.

FIELD OF THE INVENTION

The present invention relates generally to a transmission for serieshydraulic hybrid vehicles in which the transmission shifting utilizesspeed and/or torque dither.

BACKGROUND OF THE INVENTION

Vehicle transmissions are well known and include automatic, manual andsemi-automatic types. A trend in the industry has been to make vehiclesmore efficient to reduce operation costs and pollution. Accordingly,hybrid vehicles having two separate power sources have becomeincreasingly popular. A typical hybrid includes an internal combustionengine as one of its power sources, while the second power source may bean electric motor or hydrostatic transmission system. In eitherarrangement, the primary and secondary power sources are typicallyconnected to the vehicles drivetrain, each being capable of providingpower to the wheels of the vehicle either separately or in tandem.

One type of hybrid vehicle system includes an internal combustionengine, a reversible, variable displacement hydraulic motor/pump whichcan be driven by the internal combustion engine, an energy accumulatorsupplied by said motor/pump, and at least one reversible hydraulicdriving motor for propelling the vehicle that is supplied by thehydraulic accumulator and/or by said motor/pump operating as a pump andbeing driven by the engine. A transmission unit connects the engine withthe reversible, variable displacement hydraulic motor/pump during afirst mode of operation (city mode) and connects the engine to thevehicle drive wheel during a second mode of operation (highway mode).

In a city driving mode, the hydraulic accumulator is rechargedintermittently and automatically by the reversible hydraulic motor/pumpoperating as a pump driven by the engine.

Between two charging stages, the internal combustion engine can be shutdown to conserve fuel. When the pressure of the accumulator falls belowa set value, the engine is started by the hydraulic motor/pump operatingas a motor that is supplied pressurized fluid from the accumulator. Theuse of such accumulators and reversible hydraulic, variable displacementmotor/pumps in hybrid vehicles is well known in the art. See, forexample, U.S. Pat. No. 4,242,922.

When the driver of the vehicle depresses the accelerator pedal, theaccumulator discharges into the reversible hydraulic driving motor ormotors operating as such as to propel the vehicle. When the driverapplies the vehicle brakes, the reversible hydraulic motor or motorsoperate as a pump run by the rotation of the moving vehicles wheels and,therefore, serve to recover part of the kinetic or potential energy ofthe moving vehicle in order to recharge the accumulator.

For highway driving conditions, a second mode of operation may beselected wherein the transmission connects the engine to the vehicledrive wheels and the engine continuously drives the transmission in aconventional manner. In this mode, the reversible hydraulic motor/pumpsare stopped.

Such systems provide reduced fuel consumption, noise and atmosphericpollution under city traffic conditions as a result of energy recoveryduring brake application and as a result of intermittent operation ofthe engine. In addition, in low speed range of city mode the driver usesonly an accelerator and a brake without the need to shift gears sincethe variable displacement hydraulic motor/pump provides continuouslyvariable power transmission. Finally, the fuel consumption of thevehicle on highways corresponds to that of a conventional vehicle sincethe engine is coupled to the drive wheels in a conventional manner.

Shifting the transmission from the city mode to the highway modetypically involves decoupling the engine from the reversible variabledisplacement pump/motor and coupling the motor directly to the drivewheels. Conversely, shifting from highway mode to city mode typicallyinvolves decoupling the engine from the drive wheels and coupling theengine to the reversible variable displacement pump/motor.

A smooth transition between city and highway modes and different gearratios within city mode is often sought so as to minimize joltingtransmission components and to provide a more comfortable ride for theoccupants of the vehicle. Torque dithering of the engine output duringshifting has been used in the past to achieve smoother shifting. Torquedithering generally includes varying the torque output of the motorabout a desired torque value so as to avoid issues such as gear toothbutting and/or jerky shifts. For example, it is known to modulate thetorque output of an engine by controlling its fuel supply to achieve adesired torque output.

SUMMARY OF THE INVENTION

A system and method of dithering is provided that offers quicker,smoother shifts, and can increase efficiency. The system utilizes storedhydraulic energy to dither the output of a variable reversiblepump/motor unit in order to achieve quick and smooth shifts between cityand highway mode, or between various ranges within the city mode.

Accordingly, a method of engaging a transmission of a series hydraulichybrid system having at least one pump/motor selectively connectable toan output drive shaft of the transmission comprises the steps ofcalculating a desired speed of an output shaft of the at least onepump/motor, operating the at least one pump/motor at a target speed thatis the desired output shaft speed plus a desired speed dither, andengaging the output shaft of the pump/motor with the output shaft of thetransmission when the target speed is reached within a prescribed speederror threshhold. The operating step includes regulating the flow offluid to and from an accumulator to the pump/motor to achieve the targetspeed. The engaging the transmission can include engaging a gearassociated with the output shaft of the motor with a gear associatedwith the output drive shaft of the transmission. The calculating caninclude measuring the output drive shaft speed of the transmission.

The method may further comprise the steps of (i) selecting a gear from aplurality of different ratio gears associated with the output shaft ofthe pump/motor, and engaging the selected gear with a gear associatedwith the output drive shaft of the transmission, (ii) selecting a gearratio corresponding to a first gear set of a plurality of gear setshaving different ratios associated with the output shaft of thepump/motor, and engaging a clutch associated with the output drive shaftof the transmission to thereby engage the output shaft of the pump/motorwith the output drive shaft of the transmission with the selected gearratio, and/or (iii) engaging the output shaft of the pump/motor to theoutput drive shaft of the transmission.

The disengaging can include calculating a desired torque of the outputshaft of the at least one pump/motor, operating the at least onepump/motor at a target torque that is the desired torque plus a desiredtorque dither, and disengaging the output shaft of the at least onepump/motor with the output shaft of the transmission when the targettorque is commanded. The operating can include regulating the flow offluid to and from an accumulator to the pump/motor to achieve the targettorque.

According to another aspect, a method of disengaging a transmission of aseries hydraulic hybrid system having at least one pump/motorselectively connectable to an output drive shaft of the transmission,comprises calculating a desired torque of an output shaft of the atleast one pump/motor, operating the at least one pump/motor at a targettorque that is the desired torque plus a desired torque dither, anddisengaging the output shaft of the pump/motor with the output shaft ofthe transmission when the target torque is commanded. The operating caninclude regulating the flow of fluid to and from an accumulator to thepump/motor to achieve the target torque. The disengaging the outputshaft of the pump/motor from the output drive shaft can includedisengaging a gear associated with the output shaft of the motor withthe output drive shaft of the transmission, and the calculating caninclude include measuring the torque of the output drive shaft of thetransmission.

According to another aspect, a hybrid transmission system for a vehiclecomprises a primary hydraulic pump/motor, a secondary hydraulicpump/motor connected to the primary hydraulic pump/motor via a highpressure manifold, and an accumulator for storing pressurized fluidconnected to both the primary and secondary pump/motors via the highpressure manifold. The transmission has an input shaft for receivingpower from a prime mover, an output drive shaft for providing power to adrive element of the vehicle, power transmission components forselectively coupling the input shaft to at least one of the primaryhydraulic pump and the output drive shaft, and for selectively couplingthe secondary hydraulic pump/motor to the output drive shaft, and acontroller for calculating a desired speed of an output shaft of thesecondary pump/motor, operating the secondary pump/motor at a targetspeed that is the desired speed plus a desired speed dither, andengaging the output shaft of the secondary pump/motor with the outputdrive shaft of the transmission when the target speed is reached withina prescribed speed error threshold, and wherein the controller controlsthe high pressure manifold to supply fluid from the accumulator to thesecondary pump/motor for operating the secondary pump/motor at thetarget speed.

The power transmission components can include a plurality of gear setsassociated with the output shaft of the secondary pump/motor, each ofthe gear sets having a different gear ratio and being separatelycouplable to the output drive shaft of the transmission via a clutch.The clutch can be operable to either engage a gear set of the pluralityof gear sets to couple the output shaft of the pump/motor with theoutput drive shaft of the transmission, or to disengage the output shaftof the pump motor from the output drive shaft. The transmission can beincluded in a vehicle having a prime mover, and at least one driveelement coupled to the output drive shaft of the transmission forpropelling the vehicle. The drive element can include a wheel, and theprime mover can include an internal combustion engine.

According to another aspect, a hybrid transmission system for a vehiclecomprises a primary hydraulic pump/motor, a secondary hydraulicpump/motor connected to the primary hydraulic pump/motor via a highpressure manifold; and an accumulator for storing pressurized fluidconnected to both the primary and secondary pump/motors via the highpressure manifold. The transmission has an input shaft for receivingpower from a prime mover, an output drive shaft, power transmissioncomponents for selectively coupling the input shaft to at least one ofthe primary hydraulic pump and the output drive shaft and forselectively coupling the secondary hydraulic pump/motor to the outputdrive shaft, and a controller for calculating a desired torque of anoutput shaft of the secondary pump/motor, operating the secondarypump/motor at a target torque that is the desired torque plus a desiredtorque dither, and disengaging the output shaft of the secondarypump/motor from the output drive shaft of the transmission when thetarget torque is commanded. The controller regulates the high pressuremanifold to supply fluid to and from an accumulator to the pump/motor toachieve the target torque.

The power transmission components can include a plurality of gear setsassociated with the output shaft of the secondary pump/motor, each ofthe gear sets having a different gear ratio and being separatelycouplable to the output drive shaft of the transmission via a clutch.The clutch can be operable to either engage a gear set of the pluralityof gear sets to couple the output shaft of the pump/motor with theoutput drive shaft of the transmission, or to disengage the output shaftof the pump motor from the output drive shaft. A vehicle including thehybrid transmission and a prime mover is provided, with the prime movercoupled to the input shaft of the transmission. The vehicle can includeat least one drive element coupled to the output drive shaft of thetransmission for propelling the vehicle.

Further features of the invention will become apparent from thefollowing detailed description when considered in conjunction with thedrawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic diagram of a hybrid transmission system includingan exemplary power drive unit in accordance with the invention.

FIG. 2 is a schematic diagram of the exemplary power drive unit in aneutral configuration in accordance with the invention.

FIG. 3 is a schematic diagram of the exemplary power drive unit in alow-range hydrostatic configuration in accordance with the invention.

FIG. 4 is a schematic diagram of the exemplary power drive unit in abraking/accumulator charging configuration in accordance with theinvention.

FIG. 5 is a schematic diagram of the exemplary power drive unit in ahigh-range hydrostatic configuration in accordance with the invention.

FIG. 6 is a schematic diagram of the exemplary power drive unit in adirect drive configuration in accordance with the invention.

FIG. 7 is a flowchart illustrating an exemplary method of torquedithering in accordance with the invention.

FIG. 8 is a flowchart illustrating an exemplary method of speeddithering in accordance with the invention.

DETAILED DESCRIPTION

Turning now to the drawings, and initially to FIG. 1, an exemplaryhydraulic hybrid system 20 in accordance with the invention isillustrated schematically. Although the invention will be described inthe context of a hydraulic hybrid system, it will be appreciated thataspects of the invention can be applied to a wide range of powertransmission devices.

The system 20 generally includes a power drive unit 24 (transmission)having an input 26 for receiving rotational power from a prime mover 30,such as an internal combustion engine (not shown), for example. Thepower drive unit 24 includes a primary pump/motor 34 fluidly connectedto a pair of secondary pump/motors 38 via high pressure manifold 42.

An accumulator 46 for storing pressurized fluid is connected to both theprimary and secondary pump/motors 34 and 38 via high pressure manifold42. As will be described, high pressure manifold 42 serves to supplypressurized fluid to one or more of the primary and secondarypump/motors 34 and 38 and accumulator 46 depending on the mode ofoperation of the system 20. The system 20 also includes a low pressurefluid cooler and/or charge pump 50 that supplies charge fluid from a lowpressure sump or reservoir 54 to the pumps/motors, and circulates fluidto an oil cooler 58.

As will be described in greater detail below, the system 20 includespower transmission elements (e.g., gears, clutches, etc.) for (i)selectively connecting the prime mover 30 to an output shaft 44 fordirectly driving wheels of a vehicle, (ii) selectively connecting theprimary pump/motor 34 to the input shaft 26 for either pumping fluid tothe secondary pump/motors 38 and/or accumulator 46, or for providingrotational power to start the prime mover 30, and (iii) selectivelyconnecting the secondary pump/motors 38 to the output shaft for drivingwheels of a vehicle and/or, when used in a regenerative brakingconfiguration, for supplying rotational input to the secondarypump/motors 38 for charging the accumulator. All of these variousoperations are controlled by an electronic control system (ECS) 62 thatreceives inputs from various sources including operator inputs such asan accelerator pedal, brake pedal, gear select, etc.

With further reference to FIGS. 2-6, the details of the exemplary powerdrive unit 24 are illustrated. As will be appreciated, the power driveunit 24 includes input shaft 26 for connection to the prime mover,primary pump/motor 34, the pair of secondary pump/motors 38, and lubepump 50. Power transmission elements 68 in the form of shafts, gear, andclutches cooperate to perform the various operations enumerated above.

In particular, the power transmission elements include a clutch C1 forselectively coupling the primary pump/motor 34 to the input shaft 26 viagears Z1 and Z2, clutch C2 for selectively coupling the input shaft 26to the output shaft 44 for direct drive, and clutch C3 for selectivelycoupling the secondary pump/motors 38 to the output shaft 44 via gear Z7and Z4/Z6 (low range) or gears Z7/Z9. With the forgoing in mind, theoperation of the transmission 24 will now be described.

In FIG. 2, the transmission 24 is in a neutral state, with clutches C1,C2, and C3 all open (e.g. disengaged). On engine start-up, the ECS 62commands the primary pump/motor 34 to act as a pump, and the prime mover30 (also referred to as an engine) is coupled to the primary pump/motor34. The high pressure manifold 42, which includes integral valves forcontrolling flow between the primary pump/motor 34, the secondarypump/motors 38, and the accumulator system 46, directs hydraulic fluidfrom the primary pump/motor 34 to the accumulator system 46 to build upa controlled volume of hydraulic fluid under pressure. The accumulatorsystem 46 may include a single accumulator or a bank of two or moreunits depending on the total volume of oil needed to be stored in thesystem. The fluid stored in the accumulator system 46 is also availableto supply the secondary pump/motors 38 when acting as motors for use indriving the vehicle, as will be described.

The system 20 has two modes of operation. First, in the city mode orwork cycle mode, which accommodates frequent stop and go operation atlow speeds, for example less than 40 Mph, the reversible secondaryhydraulic pump/motors 38 drive the vehicle through a multiple speed, orsingle speed, transmission. The unit of FIGS. 2-6, is a two speed unithaving two secondary pump/motor units 38, and the transmission shiftsfrom a first gear ratio to a second gear ratio during hydrostaticoperation within the city mode, as will be described.

In the first mode of operation (city mode), shown in FIG. 3, theposition of the vehicle accelerator and brake pedals are detected bysensors and act as input commands to the ECS 62. If the desired actionis to accelerate, then the ECS 62 sets the secondary hydraulic drivepump/motors 38 to act as motors and the high pressure manifold 42directs hydraulic fluid stored under pressure in the accumulator system46 to drive the secondary pump/motors 38 which are coupled to the outputdrive shaft 44 through clutch C3.

If the desired action is to decelerate or brake, then as shown in FIG.4, the ECS 62 sets the secondary pump/motors 38 to act as pumps anddeliver high pressure fluid back through the high pressure manifold 42into the accumulator system 46. The secondary pump/motors 38, coupled tothe output drive shaft 44 by clutch C3 and acting as pumps, generateresistance in the drive train to slow the vehicle down. This action alsorecovers most of the kinetic energy from the vehicle and stores it forfuture use by the drive system or for performing other hydraulic poweredwork related tasks on the vehicle.

In this braking mode, mechanical brakes of the vehicle are not normallyneeded to decelerate the vehicle, but they are available for use if thebraking force required (such as in an emergency stop) is greater thanthat which is being generated by the secondary pump/motors 38 acting aspumps or as a back-up in case of a failure in the hydraulic drive system20.

If the pressure level or other sensor input indicates that theaccumulator system 46 is fully charged, then the ECS 62 can disengageclutch C1 thereby decoupling the engine from the primary pump/motor 34,and shut the engine off to conserve fuel until additional power isneeded.

The stored fluid in the accumulator system 46 can then be used for stopand go operation in the city mode with the engine off until theaccumulator system 46 signals the ECS 62 that it is getting low on itsfluid charge and needs to be refilled. At this point, the ECS 62 setsthe primary pump/motor 34 to reverse and act as a motor, and couples theprimary pump/motor 34 and engine 30 by engaging clutch C1. The ECS 62then directs the high pressure manifold 42 to send high pressure fluidfrom the accumulator system 46 to the primary pump/motor 34 and, withthe clutch C1 engaged, the primary pump/motor 34 is used to restart theengine 30. Once the engine 30 is started, the primary pump/motor 34 isagain reversed to act as a pump driven by the engine and directs highpressure fluid back through the high pressure manifold 42 into theaccumulator system 46 for replenishment. This sequence can repeatcontinuously during city mode resulting in significant savings in fuelconsumption by the engine 30.

In the illustrated embodiment, the city mode uses a two speed mechanicalgear ratio set driven by the secondary pump/motors 38 so that thesemotors can be operated within their most efficient speed ranges. A lowrange city mode configuration is illustrated in FIG. 3, while a highrange city mode configuration is illustrated in FIG. 5. As an example,the low range may provide vehicle speeds from 0 to about 25 Mph, and thesecond gear, or high range, may provide vehicle speeds from about 25 to40 Mph. The selection of the preferred shifting point can be set by ECSsoftware or can be manually selected by the operator depending upondesired duty cycle and operating conditions. These shift points do nothave to be speed related but can be modified or controlled by othersensor inputs such as vehicle incline angle, gross loaded weight,ambient temperature, hydraulic fluid temperature, or other performanceinfluencing factors, for example.

Once the vehicle has accelerated past the top speed of the city modesetting, for example about 40 MPH, the ECS 62 commands the transmissionto shift into highway mode utilizing engine 30 to directly drive theoutput drive shaft 44 by engaging clutch C2 engaged as shown in FIG. 6.

In this mode, the engine 30 will be running within its most efficientspeed range and best fuel economy. The primary and secondary pump/motors34 and 38 are disengaged from the drive train by clutch C3 set inneutral to further maximize overall vehicle efficiency.

Since both gear sets of the two secondary pump motors 38 are in constantmesh, and shifting is accomplished by the secondary clutch C3 capable ofselecting “neutral” for idle and direct drive, hydro low (gear Z4engaged) or hydro high (gear Z7 engaged), it is possible to control thetorque output and/or speed of the secondary pump/motors 38 forsynchronization to achieve a smooth shift either up or down. This can beaccomplished by using the stored hydraulic fluid from the accumulatorsystem 46 independent of the primary pump/motor 34 speed ordisplacement.

To achieve quick and smooth engagement and disengagement, and inaccordance with the present invention, speed or torque dither isintroduced to one or more of the secondary hydraulic pump/motors 38output shaft to avoid possible issues such as gear tooth butting. Thespeed or torque dither results in dither to the appropriate transmissiongears and enables a quick and smooth engagement or disengagement of thehigh and low gears Z4 and Z7 by clutch C3.

In FIG. 7, an exemplary method 108 for transmission disengagement withtorque dither of the secondary hydraulic pump/motors 38 is illustrated.The method begins at step 110 when the transmission enters disengagementstage, generally initiated automatically by the ECU 62 based on apreprogrammed shift point, or upon request by the vehicle operator. Inprocess step 112, the ECU 62 controls the secondary hydraulicpump/motors 38 to get a desired target torque. The desired torque can bedetermined, for example, by test results and/or estimations includingmeasuring pump drag torques at different speeds and temperatures. Forexample, at the beginning of transmission disengagement stage, secondarypump/motor 38 output torque is ramped down to a desired target value. Inprocess step 114, the desired torque value, plus a dither torque value,is achieved while pulling the transmission out of gear (e.g., clutch C3disengaged). The torque dither typically will be determined by testresults, and can be a preset value programmed into the ECU. By way ofexample, the secondary hydraulic pump/motor 38 output torque iscontrolled to achieve the desired target value plus a desired dithertorque while pulling transmission out of gear (e.g., disengaging clutchC3). The desired dither torque is an alternating small positive andnegative torque around 0 to eliminate mechanical friction to relievegear torque lock. In process step 116, the ECU 62 stops controlling thesecondary pump/motors 38 and exits from disengagement stage whentransmission disengagement is confirmed.

Accordingly, it will be appreciated that torque dither of the secondaryhydraulic pump/motors 38 is achieved by utilizing the pressurized fluidstored within the accumulator, rather than fluid supplied from theprimary pump/motor 34. Thus, the primary pump/motor 34, and by extensionthe prime mover 30, need not be in operation during torque dithering.

With reference to FIG. 8, an exemplary speed dither method 118 isillustrated. Like the torque dithering method 108 described above, thespeed dithering is achieved by utilizing the pressurized fluid storedwithin the accumulator to ensure smooth engagement of the gears. Inprocess step 120, the transmission enters the engagement stage. As willbe appreciated, the engagement stage may immediately follow thedisengagement of either the low gear Z4 or the high gear Z7 depending onwhether the transmission is upshifting or downshifting, for example. Inprocess step 122, a target speed setting of the secondary pump/motors 38output shaft 123 (see FIGS. 2-6) is calculated depending on transmissionoutput shaft 44 speed and desired gear ratio. In process step 124, thedetermination of whether the vehicle is stationary is made. Ifstationary, the method continues to process step 126 and the targetspeed is set to zero to two times the dither amplitude. Dither amplitudemay generally be determined by testing. For example, the ditheramplitude is normally low, and so is the dither frequency. Thus, forvehicle starting from 0 speed, the dither should be set from 0 to 2times the dither amplitude to make the target speed in the rightdirection.

If the vehicle is not stationary, the method continues to process step128, whereat the target speed for the secondary pump/motor 38 outputspeed is set to the desired secondary pump/motor 38 output shaft 123speed plus a desired dither speed. The desired dither speed is generallydetermined from test results, and is preset in the ECU 62. The secondarypump/motors 38 are then controlled at the target speed in process step130, and the transmission gears are engaged when the target speed isachieved in process step 132 thereby achieving a smooth shift. It willbe appreciated that with this technique the secondary hydraulicpump/motor 38 is used to generate speed dither for transmissionsynchronization and engagement.

While suitable valving could be used to perform both the torque andspeed dithering of the secondary pump/motors 38, one type of pump/motorthat is particularly well suited for such operations is a variable flowover-center piston pump, such as the pump described in U.S. Pat. No.4,991,492, which is hereby incorporated herein by reference in itsentirety. Such pump/motor design is capable of rapidly changing itsoperations, and thus can quickly implement the torque and/or speeddithering functions during engagement/disengagement.

Although the invention has been shown and described with respect to acertain preferred embodiment or embodiments, it is obvious thatequivalent alterations and modifications will occur to others skilled inthe art upon the reading and understanding of this specification and theannexed drawings. In particular regard to the various functionsperformed by the above described elements (components, assemblies,devices, compositions, etc.), the terms (including a reference to a“means”) used to describe such elements are intended to correspond,unless otherwise indicated, to any element which performs the specifiedfunction of the described element (i.e., that is functionallyequivalent), even though not structurally equivalent to the disclosedstructure which performs the function in the herein illustratedexemplary embodiment or embodiments of the invention. In addition, whilea particular feature of the invention may have been described above withrespect to only one or more of several illustrated embodiments, suchfeature may be combined with one or more other features of the otherembodiments, as may be desired and advantageous for any given orparticular application.

1. A method of engaging a transmission of a series hydraulic hybridsystem having at least one pump/motor selectively connectable to anoutput drive shaft of the transmission, the method comprising the stepsof: calculating a desired speed of an output shaft of the at least onepump/motor; operating the at least one pump/motor at a target speed thatis the desired output shaft speed plus a desired speed dither; andengaging the output shaft of the pump/motor with the output shaft of thetransmission when the target speed is reached within a prescribed speederror threshold; wherein the operating includes regulating the flow offluid to and from an accumulator to the pump/motor to achieve the targetspeed.
 2. A method as set forth in claim 1, wherein the engaging thetransmission includes engaging a gear associated with the output shaftof the motor with a gear associated with the output drive shaft of thetransmission.
 3. A method as set forth in claim 1, further comprisingthe step of selecting a gear from a plurality of different ratio gearsassociated with the output shaft of the pump/motor, and the engagingincludes engaging the selected gear with a gear associated with theoutput drive shaft of the transmission.
 4. A method as set forth inclaim 1, further comprising the step of selecting a gear ratiocorresponding to a first gear set of a plurality of gear sets havingdifferent ratios associated with the output shaft of the pump/motor, andthe engaging includes engaging a clutch associated with the output driveshaft of the transmission to thereby engage the output shaft of thepump/motor with the output drive shaft of the transmission with theselected gear ratio.
 5. A method as set forth in claim 1, wherein thecalculating includes measuring the output drive shaft speed of thetransmission.
 6. A method as set forth in claim 1, further comprisingdisengaging the output shaft of the pump/motor from the output driveshaft of the transmission, the disengaging including: calculating adesired torque of the output shaft of the at least one pump/motor;operating the at least one pump/motor at a target torque that is thedesired torque plus a desired torque dither; and disengaging the outputshaft of the at least one pump/motor with the output shaft of thetransmission when the target torque is commanded; wherein the operatingincludes regulating the flow of fluid to and from an accumulator to thepump/motor to achieve the target torque.
 7. A method of disengaging atransmission of a series hydraulic hybrid system having at least onepump/motor selectively connectable to an output drive shaft of thetransmission, the method comprising the steps of: calculating a desiredtorque of an output shaft of the at least one pump/motor, operating theat least one pump/motor at a target torque that is the desired torqueplus a desired torque dither; disengaging the output shaft of thepump/motor with the output shaft of the transmission when the targettorque is commanded; wherein the operating includes regulating a flow offluid to and from an accumulator to the pump/motor to achieve the targettorque.
 8. A method as set forth in claim 7, wherein the disengaging theoutput shaft of the pump/motor from the output drive shaft includesdisengaging a gear associated with the output shaft of the motor withthe output drive shaft of the transmission.
 9. A method as set forth inclaim 7, wherein the calculating includes measuring the torque of theoutput drive shaft of the transmission.
 10. A hybrid transmission systemfor a vehicle comprising: a primary hydraulic pump/motor; a secondaryhydraulic pump/motor connected to the primary hydraulic pump/motor via ahigh pressure manifold; and an accumulator for storing pressurized fluidconnected to both the primary and secondary pump/motors via the highpressure manifold; wherein the transmission has an input shaft forreceiving power from a prime mover, an output drive shaft, powertransmission components for selectively coupling the input shaft to atleast one of the primary hydraulic pump and the output drive shaft andfor selectively coupling the secondary hydraulic pump/motor to theoutput drive shaft, and a controller for calculating a desired speed ofan output shaft of the secondary pump/motor, operating the secondarypump/motor at a target speed that is the desired speed plus a desiredspeed dither, and engaging the output shaft of the secondary pump/motorwith the output drive shaft unit when the target speed is reached withina prescribed speed error threshold; wherein the controller controls thehigh pressure manifold to supply fluid from the accumulator to thesecondary pump/motor for operating the secondary pump/motor at thetarget speed.
 11. A hybrid transmission system as set forth in claim 10,wherein the power transmission components include a plurality of gearsets associated with the output shaft of the secondary pump/motor, eachof the gear sets having a different gear ratio and being separatelycouplable to the output drive shaft of the transmission via a clutch.12. A hybrid transmission system as set forth in claim 11, wherein theclutch is operable to either engage a gear set of the plurality of gearsets to couple the output shaft of the pump/motor with the output driveshaft of the transmission, or to disengage the output shaft of the pumpmotor from the output drive shaft.
 13. A vehicle comprising a primemover and the hybrid transmission system as set forth in claim 10,wherein the prime mover is coupled to the input shaft of thetransmission.
 14. A vehicle as set forth in claim 13, further comprisingat least one drive element coupled to the output drive shaft of thetransmission for propelling the vehicle.
 15. A vehicle as set forth inclaim 14, wherein the at least one drive element includes a wheel.